Exhaust Mechanism

ABSTRACT

An exhaust mechanism includes: an exhaust pipe configured to circulate exhaust gas from an engine; a first outer pipe disposed along the exhaust pipe on an outer periphery of the exhaust pipe; a second outer pipe disposed along the first outer pipe on an outer periphery of the first outer pipe; a pair of intermediate members disposed on the outer periphery of the exhaust pipe, fixed to respective inner peripheral surfaces of a first end portion and a second end portion of the first outer pipe, and being lower in thermal conductivity than the exhaust pipe. A vacuum layer is provided between the first outer pipe and the second outer pipe. At least one of the intermediate members is configured to slide in an axial direction of the exhaust pipe.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2016-251559 filed on Dec. 26, 2016 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The disclosure relates to an exhaust mechanism.

2. Description of Related Art

Japanese Unexamined Patent Application Publication No. 11-36856 (JP 11-36856 A) discloses a structure in which an exhaust pipe between a catalytic converter and an exhaust manifold of an engine is configured as a vacuum double pipe that has an outer pipe and an inner pipe and a part of the inner pipe takes the form of a bellows pipe.

SUMMARY

In the structure disclosed in JP 11-36856 A, a decline in the temperature of the exhaust gas that circulates through the inner pipe is suppressed by the heat insulation effect of a vacuum layer between the outer pipe and the inner pipe. In the structure disclosed in JP 11-36856 A, the heat insulation effect of the vacuum layer results in a difference in temperature between the outer pipe and the inner pipe when a high-temperature exhaust gas circulates through the inner pipe during high-speed traveling of a vehicle. Then, a compressive load is applied to the inner pipe as a result of a difference in axial thermal elongation between the outer pipe and the inner pipe. The difference in the axial thermal elongation between the outer pipe and the inner pipe is absorbed by the bellows pipe being compressively deformed in an axial direction by the compressive load.

In the structure disclosed in JP 11-36856 A, first and second end portions of the outer pipe are joined by welding to the inner pipe, and thus heat transfer occurs between the outer pipe and the inner pipe in the first and second end portions of the outer pipe. Accordingly, the temperature of the exhaust gas may fall below the dew point temperature. In a case where the vacuum double pipe is short and the vacuum layer has little heat insulation effect, in particular, the temperature of the exhaust gas is likely to fall below the dew point temperature and condensed water is likely to be generated. Once the condensed water is generated, the flow path area of the exhaust pipe decreases due to the accumulation of the condensed water, and then an increase in flow resistance and a decline in engine output arise.

The disclosure provides an exhaust pipe mechanism capable of suppressing a decline in the temperature of exhaust gas attributable to heat transfer in first and second end portions of an outer pipe.

An aspect of the present disclosure is an exhaust mechanism. The exhaust mechanism includes: an exhaust pipe configured to circulate exhaust gas from an engine; a first outer pipe disposed along the exhaust pipe on an outer periphery of the exhaust pipe; a second outer pipe disposed along the first outer pipe on an outer periphery of the first outer pipe; and a pair of intermediate members disposed on the outer periphery of the exhaust pipe, fixed to respective inner peripheral surfaces of a first end portion and a second end portion of the first outer pipe, and being lower in thermal conductivity than the exhaust pipe. A vacuum layer is provided between the first outer pipe and the second outer pipe. At least one of the intermediate members is configured to slide in an axial direction of the exhaust pipe.

In the exhaust pipe mechanism described above, the exhaust pipe allows the exhaust gas from the engine to circulate. The first outer pipe is disposed along the axial direction of the exhaust pipe on the outer periphery of the exhaust pipe. The second outer pipe is disposed along the axial direction of the first outer pipe on the outer periphery of the first outer pipe. The vacuum layer is formed between the first outer pipe and the second outer pipe. Heat dissipation from the inner peripheral side of the first outer pipe to the outer peripheral side of the first outer pipe is suppressed by the heat insulation effect of the vacuum layer.

In the exhaust pipe mechanism described above, the intermediate members are disposed on the outer periphery of the exhaust pipe and fixed to the respective inner peripheral surfaces of the first end portion and the second end portion of the first outer pipe. The intermediate members are lower in thermal conductivity than the exhaust pipe. Accordingly, heat transfer between the exhaust pipe and the first and second end portions of the first outer pipe can be more suppressed than in a structure in which the first and second end portions of the first outer pipe are joined by welding to the exhaust pipe. As a result, a decline in the temperature of the exhaust gas attributable to heat transfer in the first and second end portions of the first outer pipe can be suppressed.

When a high-temperature exhaust gas circulates through the exhaust pipe during high-speed traveling of a vehicle, for example, the heat insulation effect of the intermediate members results in a difference in temperature between the exhaust pipe and the first outer pipe and a difference in axial thermal elongation between the exhaust pipe and the first outer pipe in the exhaust pipe mechanism. The difference in the axial thermal elongation between the exhaust pipe and the first outer pipe can be absorbed by at least one of the intermediate members sliding in the axial direction of the exhaust pipe.

“The intermediate members being lower in thermal conductivity than the exhaust pipe” includes a configuration in which a material with a thermal conductivity lower than the thermal conductivity of the material of the exhaust pipe is used for the intermediate members and a configuration in which the intermediate members have a reduced contact area by having a plurality of voids.

The exhaust pipe mechanism may further include a first low-permeability layer disposed on at least one of an inner peripheral surface and an outer peripheral surface of the first outer pipe, and being made of a material with a hydrogen permeability lower than a hydrogen permeability of the first outer pipe and a second low-permeability layer disposed on at least one of an inner peripheral surface and an outer peripheral surface of the second outer pipe, and being made of a material with a hydrogen permeability lower than a hydrogen permeability of the second outer pipe.

In the exhaust pipe mechanism, hydrogen permeation from the atmosphere to a vacuum layer through the second outer pipe and the first outer pipe is suppressed by the first low-permeability layer and the second low-permeability layer. Accordingly, rising of the pressure of the vacuum layer above the partial pressure of hydrogen in the atmosphere is suppressed. As a result, the heat insulation effect of the vacuum layer can be maintained.

The first low-permeability layer and the second low-permeability layer may be an aluminum layer or a resin layer. With the above-described configuration, hydrogen permeation from the atmosphere to the vacuum layer through the second outer pipe and the first outer pipe is suppressed. Accordingly, rising of the pressure of the vacuum layer above the partial pressure of hydrogen in the atmosphere is suppressed. As a result, the heat insulation effect of the vacuum layer can be maintained.

The pair of intermediate members may be made of a material that is at least one of glass wool, metallic mesh, and ceramic. With the above-described configuration, the heat insulation effect of the intermediate members can be obtained since the intermediate members are made of a material that is lower in thermal conductivity than the exhaust pipe.

By being configured as described above, the disclosure is capable of having an excellent effect in suppressing a decline in the temperature of the exhaust gas attributable to heat transfer in the first and second end portions of the outer pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a perspective view illustrating an exhaust pipe structure according to a first embodiment;

FIG. 2 is a side view illustrating the exhaust pipe structure according to the first embodiment;

FIG. 3 is a side sectional view illustrating a triple pipe according to the first embodiment;

FIG. 4 is a partially enlarged side sectional view illustrating a part of the triple pipe illustrated in FIG. 3;

FIG. 5 is a side sectional view of an exhaust pipe structure according to a comparative example and a graph illustrating the gas temperature of exhaust gas circulating through an exhaust pipe of the exhaust pipe structure;

FIG. 6 is a side sectional view of the triple pipe according to the first embodiment and a graph illustrating the gas temperature of exhaust gas circulating through the triple pipe;

FIG. 7 is a side sectional view illustrating the exhaust pipe structure according to the first embodiment to which intermediate members according to a modification example is applied;

FIG. 8 is a side sectional view illustrating an exhaust pipe structure according to a second embodiment;

FIG. 9 is a side sectional view of a triple pipe according to the second embodiment and a graph illustrating the gas temperature of exhaust gas circulating through the triple pipe; and

FIG. 10 is a side sectional view illustrating a modification example of the exhaust pipe structure according to the second embodiment in which low-permeability layers are disposed on the inner and outer peripheral surfaces of both first and second outer pipes.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, examples of embodiments of the disclosure will be described based on accompanying drawings. The arrows RR, UP, and RH appropriately shown in each of the drawings represent the rear, upper, and right sides of a vehicle, respectively. In the following description, the front-rear direction of the vehicle will be simply referred to as the front and rear of the vehicle and the up-down direction of the vehicle will be simply referred to as the top and bottom of the vehicle in some cases.

An exhaust pipe structure 10 according to a first embodiment (example of an exhaust system) will be described first.

FIGS. 1 and 2 are a perspective view and a side view illustrating the exhaust pipe structure 10, respectively. In each of the drawings including FIGS. 1 and 2, the structure is illustrated in a simplified manner for easy understanding of the exhaust pipe structure 10 according to the present embodiment.

The exhaust pipe structure 10 is a pipe structure for discharging the exhaust gas that is discharged from the engine (not illustrated) of the vehicle such as an automobile to the atmosphere (to the outside of the vehicle). Specifically, the exhaust pipe structure 10 is provided with a first exhaust pipe 11, a second exhaust pipe 20, a main muffler 40, and a discharge pipe 50 as illustrated in FIG. 1.

As illustrated in FIG. 1, the first exhaust pipe 11 is configured as a pipe that extends along the front-rear direction of the vehicle. The first exhaust pipe 11 has a front end portion that is connected to the engine (not illustrated) of the vehicle. As a result, the exhaust gas from the engine flows in from the front end portion of the first exhaust pipe 11 and circulates to the rear side of the vehicle (to the rear end portion of the first exhaust pipe 11).

A catalytic converter 14, an exhaust heat recovery unit 16, and a submuffler 18 are disposed on the first exhaust pipe 11 in this order from the front side of the vehicle. The catalytic converter 14 has a function to purify the exhaust gas by removing certain substances from the exhaust gas passing through the catalytic converter 14.

The exhaust heat recovery unit 16 has a function to recover the heat of the exhaust gas and reuse the heat by performing heat exchange with a heat medium such as water. The submuffler 18 has a function to reduce the exhaust sound of the exhaust gas.

As illustrated in FIG. 1, the second exhaust pipe 20 is configured as a pipe that extends along the front-rear direction of the vehicle. The second exhaust pipe 20 has a front end portion that communicates with the rear end portion of the first exhaust pipe 11. As a result, the exhaust gas from the first exhaust pipe 11 flows in from the front end portion of the second exhaust pipe 20 and circulates to the rear side of the vehicle (to the rear end portion of the second exhaust pipe 20). The configuration of the second exhaust pipe 20 will be described in detail later.

As illustrated in FIG. 2, the main muffler 40 is disposed behind and above the second exhaust pipe 20 in the vehicle. The rear end portion of the second exhaust pipe 20 communicates with the main muffler 40. As a result, the exhaust gas flows into the main muffler 40 from the second exhaust pipe 20. The main muffler 40 has a function to reduce the exhaust sound of the exhaust gas flowing into the main muffler 40.

As illustrated in FIG. 1, the discharge pipe 50 extends from the main muffler 40 to the right side of the vehicle and is curved to the rear side of the vehicle. The discharge pipe 50 allows the exhaust gas from the main muffler 40 to be discharged to the atmosphere.

Specifically, the second exhaust pipe 20 has an upstream pipe 25, a downstream pipe 27, and a triple pipe 23 as illustrated in FIG. 2. The upstream pipe 25 constitutes the upstream side part of the second exhaust pipe 20. The downstream pipe 27 constitutes the downstream side part of the second exhaust pipe 20. The triple pipe 23 is disposed between the upstream pipe 25 and the downstream pipe 27.

As illustrated in FIG. 2, the upstream pipe 25 has an inclined portion 25A and a horizontal portion 25B. The inclined portion 25A has a falling gradient falling toward the rear of the vehicle. The horizontal portion 25B extends along the front-rear direction of the vehicle. The upstream pipe 25 communicates with the first exhaust pipe 11 by the front end portion of the inclined portion 25A being connected to the rear end portion of the first exhaust pipe 11. The horizontal portion 25B has a front end portion that communicates with the rear end portion of the inclined portion 25A.

The downstream pipe 27 has a horizontal portion 27A and an inclined portion 27B. The horizontal portion 27A extends along the front-rear direction of the vehicle. The inclined portion 27B has a rising gradient rising toward the rear of the vehicle. The horizontal portion 27A has a rear end portion that communicates with the front end portion of the inclined portion 27B. The downstream pipe 27 communicates with the main muffler 40 by the rear end portion of the inclined portion 27B being connected to the main muffler 40.

The triple pipe 23 is disposed below a floor panel 17 of the vehicle. Specifically, the triple pipe 23 is disposed below a fuel tank 19 as a protruding portion protruding downward from the floor panel 17 and constitutes the lowermost part of the exhaust pipe in the exhaust pipe structure 10 (its part that is disposed on the lowermost side).

Specifically, the triple pipe 23 has an inner pipe 70 (example of an exhaust pipe), a first outer pipe 61, and a second outer pipe 62 as illustrated in FIG. 3. The inner pipe 70 extends along the front-rear direction of the vehicle. The first outer pipe 61 is disposed along the axial direction of the inner pipe 70 on the outer periphery of the inner pipe 70. The second outer pipe 62 is disposed along the axial direction of the first outer pipe 61 on the outer periphery of the first outer pipe 61. The inner pipe 70, the first outer pipe 61, and the second outer pipe 62 are formed of, for example, stainless steel.

The inner pipe 70 is configured as a cylindrical circular pipe. The inner pipe 70 communicates with the upstream pipe 25 with the front end portion (first end portion) of the inner pipe 70 connected to the rear end portion of the horizontal portion 25B of the upstream pipe 25. In addition, the inner pipe 70 communicates with the downstream pipe 27 with the rear end portion (second end portion) of the inner pipe 70 connected to the front end portion of the horizontal portion 27A of the downstream pipe 27. As a result, the inner pipe 70 allows the exhaust gas from the engine (not illustrated) to circulate to the downstream pipe 27 (rear side of the vehicle).

The first outer pipe 61 is configured as a cylindrical circular pipe. As illustrated in FIG. 3, a gap is provided between the first outer pipe 61 and the inner pipe 70. In other words, the inner peripheral surface of the first outer pipe 61 is spaced apart from the outer peripheral surface of the inner pipe 70. The space that is between the first outer pipe 61 and the inner pipe 70 and is between a pair of intermediate members 80 (described later) has an atmospheric pressure.

As illustrated in FIGS. 3 and 4, the inner diameter of a front end portion 61F and a rear end portion 61R of the first outer pipe 61 (both end portions of the first outer pipe 61 in the axial direction) is larger than the inner diameter of the middle side of the first outer pipe 61 in the axial direction. Protruding portions 63 protruding radially inward are formed at the front end and the rear end of the first outer pipe 61 (both ends of the first outer pipe 61 in the axial direction). The protruding portions 63 are disposed along the circumferential direction of the first outer pipe 61.

The second outer pipe 62 is configured as a cylindrical circular pipe. As illustrated in FIGS. 3 and 4, the inner diameter of a front end portion 62F and a rear end portion 62R of the second outer pipe 62 (both end portions of the second outer pipe 62 in the axial direction) is smaller than the inner diameter of the middle side of the second outer pipe 62 in the axial direction. The front end portion 62F and the rear end portion 62R are joined by welding or the like to the front end portion 61F and the rear end portion 61R of the first outer pipe 61. As a result, a vacuum layer 90 is formed between the first outer pipe 61 and the second outer pipe 62. The internal pressure of the vacuum layer 90 is at least lower than the atmospheric pressure and is set to, for example, a pressure of approximately 10³ Pa. The first outer pipe 61 and the second outer pipe 62 are not in contact with each other at the part where the vacuum layer 90 is formed.

As illustrated in FIG. 3, the triple pipe 23 has the intermediate members 80 that are disposed on the outer periphery of the inner pipe 70 and are fixed to the respective inner peripheral surfaces of the front end portion 61F (example of a first end portion) and the rear end portion 61R (example of a second end portion) of the first outer pipe 61. Specifically, the respective outer peripheral surfaces of the intermediate members 80 are fixed to the inner peripheral surfaces of the front end portion 61F and the rear end portion 61R of the first outer pipe 61. The respective end faces of the intermediate members 80 are fixed to the protruding portions 63 (refer to FIG. 4).

Each of the intermediate members 80 is formed in an annular shape along the circumferential direction of the inner pipe 70 and the first outer pipe 61. Each of the intermediate members 80 is not fixed to the inner pipe 70 and is capable of sliding with respect to the inner pipe 70 in the axial direction of the inner pipe 70. As a result, the first outer pipe 61 is supported to be capable of sliding with respect to the inner pipe 70.

Each of the intermediate members 80 is configured as, for example, a stainless steel mesh that has a plurality of voids. The stainless steel mesh is formed by, for example, plain weaving and twill weaving being performed on stainless steel wires. The intermediate members 80 have reduced contact area by having the voids as described above. In this manner, the intermediate members 80 is shaped such that it is less likely to transfer heat than the void-less inner pipe 70 and the intermediate members 80 geometrically has a thermal conductivity lower than that of the inner pipe 70.

“The intermediate members 80 being lower in thermal conductivity than the inner pipe 70” includes a configuration in which a material with a thermal conductivity lower than the thermal conductivity of the material of the inner pipe 70 is used for the intermediate members 80 (configuration in which the thermal conductivity is lower materially) as well as a configuration in which the intermediate members 80 have the reduced contact area by having the voids (configuration in which the thermal conductivity is lower geometrically).

The action and effect of the exhaust pipe structure 10 will be described below.

The exhaust pipe structure 10 allows the exhaust gas discharged from the engine (not illustrated) of the vehicle to be discharged to the atmosphere through the first exhaust pipe 11, the second exhaust pipe 20, the main muffler 40, and the discharge pipe 50 (refer to FIG. 1).

In the exhaust pipe structure 10, the vacuum layer 90 is formed between the first outer pipe 61 and the second outer pipe 62 of the triple pipe 23 as illustrated in FIG. 3. Heat dissipation from the inner peripheral side of the first outer pipe 61 to the outer peripheral side of the first outer pipe 61 is suppressed by the heat insulation effect of the vacuum layer 90.

In the exhaust pipe structure 10, the intermediate members 80 are lower in thermal conductivity than the inner pipe 70. Accordingly, heat transfer between the inner pipe 70 and the front end portion 61F and the rear end portion 61R of the first outer pipe 61 can be more suppressed than in a structure in which the front end portion 61F and the rear end portion 61R of the first outer pipe 61 are joined by welding to the inner pipe 70. As a result, a decline in the temperature of the exhaust gas attributable to heat transfer in the front end portion 61F and the rear end portion 61R of the first outer pipe 61 can be suppressed.

In a structure in which a front end portion 160F and a rear end portion 160R of an outer pipe 160 are joined by welding to an inner pipe 170 as illustrated in FIG. 5 (comparative example), heat transfer occurs between the outer pipe 160 and the inner pipe 170 in the front end portion 160F and the rear end portion 160R of the outer pipe 160. Accordingly, the temperature of the exhaust gas may fall below the dew point temperature as shown in the graph in FIG. 5.

In the exhaust pipe structure 10, in contrast, a decline in the temperature of the exhaust gas circulating through the inner pipe 70 is suppressed by the heat insulation effect of the vacuum layer 90 and the intermediate members 80 as described above, and thus a decline in the temperature of the exhaust gas below the dew point temperature is suppressed as shown in the graph in FIG. 6.

As a result, condensation of the water vapor that is contained in the exhaust gas is unlikely to occur in the inner pipe 70 and the generation of condensed water that is attributable to the condensation is suppressed. The solid line in the graph in FIG. 6 represents the gas temperature of the exhaust gas in the structure according to the present embodiment and the dashed line in the graph in FIG. 6 represents the gas temperature of the exhaust gas in the structure according to the comparative example (structure shown in FIG. 5).

The generation of condensed water can be suppressed in the exhaust pipe structure 10 as described above. Accordingly, a decrease in the flow path area of the second exhaust pipe 20 and a decline in the anti-rust performance of the second exhaust pipe 20 attributable to the accumulation of condensed water can be effectively suppressed. Accordingly, an increase in flow resistance that is attributable to a decrease in the flow path area of the second exhaust pipe 20 is suppressed and effects such as a decline in engine output are suppressed. By a decline in the anti-rust performance of the second exhaust pipe 20 being suppressed, a low-rust resistance material and a thin plate material can be used as the material of the second exhaust pipe 20.

When a high-temperature exhaust gas circulates through the inner pipe 70 during high-speed traveling of the vehicle, for example, the heat insulation effect of the intermediate members 80 results in a difference in temperature between the inner pipe 70 and the first outer pipe 61 and a difference in axial thermal elongation between the inner pipe 70 and the first outer pipe 61 in the exhaust pipe structure 10. The difference in the axial thermal elongation between the inner pipe 70 and the first outer pipe 61 can be absorbed by the intermediate members 80 sliding in the axial direction of the inner pipe 70. In the exhaust pipe structure 10, the intermediate members 80 slide in the axial direction of the inner pipe 70 as described above, and thus the difference in the axial thermal elongation between the inner pipe 70 and the first outer pipe 61 can be absorbed. Accordingly, a structure for thermal elongation absorption, examples of which include a part of the inner pipe 70 formed in a bellows shape, is not needed.

In the exhaust pipe structure 10 according to the modification example of the first embodiment, the stainless steel mesh is used as an example of the intermediate members. However, the disclosure is not limited thereto and another metallic mesh may be used instead. For example, the intermediate members may be intermediate members 180 using glass wool as illustrated in FIG. 7. In addition, ceramic or the like may be used as the intermediate members and various materials can be used as the intermediate members.

In the exhaust pipe structure 10, both of the intermediate members 80 are capable of sliding in the axial direction of the inner pipe 70. In an alternative structure, one of the intermediate members 80 may be fixed to the inner pipe 70. In other words, at least one of the intermediate members 80 may be capable of sliding in the axial direction of the inner pipe 70.

Hereinafter, an exhaust pipe structure 200 according to a second embodiment will be described. The same reference numerals will be appropriately used to refer to its parts that have the same functions as in the first embodiment, and description thereof will be omitted as appropriate.

As illustrated in FIG. 8, the exhaust pipe structure 200 is provided with a first low-permeability layer 210 disposed on an outer peripheral surface 61A of a first outer pipe 61 and a second low-permeability layer 220 disposed on an inner peripheral surface 62B of a second outer pipe 62.

The first low-permeability layer 210 is formed of a material that is lower in hydrogen permeability than the material of the first outer pipe 61. The second low-permeability layer 220 is formed of a material that is lower in hydrogen permeability than the material of the second outer pipe 62. Specifically, each of the first low-permeability layer 210 and the second low-permeability layer 220 is configured as, for example, an aluminum layer formed of aluminum lower in hydrogen permeability than the stainless steel that forms the first outer pipe 61 and the second outer pipe 62. The aluminum layers are formed by a method such as vapor deposition and thermal spraying.

Resin layers formed of a resin material lower in hydrogen permeability than the stainless steel that forms the first outer pipe 61 and the second outer pipe 62 may be used as the first low-permeability layer 210 and the second low-permeability layer 220 insofar as the temperatures of the first outer pipe 61 and the second outer pipe 62 do not reach a heat resistance temperature as a result of the action of a pair of intermediate members 80. Examples of the resin material include polycarbonate, polypropylene (PP), and polyethylene terephthalate (PET).

In the exhaust pipe structure 200, hydrogen permeation from the atmosphere to a vacuum layer 90 through the second outer pipe 62 and the first outer pipe 61 is suppressed by the first low-permeability layer 210 and the second low-permeability layer 220. Accordingly, rising of the pressure of the vacuum layer 90 above the partial pressure of hydrogen in the atmosphere is suppressed. As a result, the heat insulation effect of the vacuum layer 90 can be maintained.

In the exhaust pipe structure 200, a decline in the temperature of the exhaust gas circulating through an inner pipe 70 is more suppressed, as shown by the one-dot chain line in the graph in FIG. 9, than in a structure that is not provided with the first low-permeability layer 210 and the second low-permeability layer 220 (as shown by the solid line).

In the exhaust pipe structure 200, the first low-permeability layer 210 is disposed on the outer peripheral surface 61A of the first outer pipe 61. In an alternative configuration, the first low-permeability layer 210 may be disposed on an inner peripheral surface 61B of the first outer pipe 61 as well as illustrated in FIG. 10. In another alternative configuration, the first low-permeability layer 210 may be disposed solely on the inner peripheral surface 61B of the first outer pipe 61. In other words, the first low-permeability layer 210 may be disposed on at least one of the outer peripheral surface 61A and the inner peripheral surface 61B of the first outer pipe 61.

In the exhaust pipe structure 200, the second low-permeability layer 220 is disposed on the inner peripheral surface 62B of the second outer pipe 62. In an alternative configuration, the second low-permeability layer 220 may be disposed on an outer peripheral surface 62A of the second outer pipe 62 as well as illustrated in FIG. 10. In another alternative configuration, the second low-permeability layer 220 may be disposed solely on the outer peripheral surface 62A of the second outer pipe 62. In other words, the second low-permeability layer 220 may be disposed on at least one of the outer peripheral surface 62A and the inner peripheral surface 62B of the second outer pipe 62.

The intermediate members in the exhaust pipe structure 200 may also be intermediate members 180 using glass wool. In addition, ceramic or the like may be used as the intermediate members and various materials can be used as the intermediate members.

One of the intermediate members 180 may be fixed to the inner pipe 70 in the exhaust pipe structure 200 as well. In other words, at least one of the intermediate members 180 may be capable of sliding in the axial direction of the inner pipe 70.

The disclosure is not limited to the embodiments described above. The disclosure can be modified, changed, and improved in various ways without departing from the scope of the disclosure. 

What is claimed is:
 1. An exhaust mechanism comprising: an exhaust pipe configured to circulate exhaust gas from an engine; a first outer pipe disposed along the exhaust pipe on an outer periphery of the exhaust pipe; a second outer pipe disposed along the first outer pipe on an outer periphery of the first outer pipe, wherein a vacuum layer is provided between the first outer pipe and the second outer pipe; and a pair of intermediate members disposed on the outer periphery of the exhaust pipe, fixed to respective inner peripheral surfaces of a first end portion and a second end portion of the first outer pipe, and being lower in thermal conductivity than the exhaust pipe, and wherein at least one of the intermediate members is configured to slide in an axial direction of the exhaust pipe.
 2. The exhaust mechanism according to claim 1, further comprising: a first low-permeability layer disposed on at least one of an inner peripheral surface and an outer peripheral surface of the first outer pipe, and being made of a material with a hydrogen permeability lower than a hydrogen permeability of the first outer pipe; and a second low-permeability layer disposed on at least one of an inner peripheral surface and an outer peripheral surface of the second outer pipe, and being made of a material with a hydrogen permeability lower than a hydrogen permeability of the second outer pipe.
 3. The exhaust mechanism according to claim 2, wherein each of the first low-permeability layer and the second low-permeability layer is one of an aluminum layer and a resin layer.
 4. The exhaust mechanism according to claim 1, wherein the pair of intermediate members is made of a material that is at least one of glass wool, metallic mesh, and ceramic. 